The present invention relates generally to implantable medical devices; and more particular, relates to implantable devices that include an exterior structure to selectively promote tissue ingrowth.
Transvenous cardioversion and defibrillation leads typically employ cardioversion or defibrillation electrodes taking the form of elongated metal coils. These coils may be applied to the exterior surface of the lead body, as disclosed in U.S. Pat. No. 4,934,049 issued to Kiekhafer et al.
One problem associated with defibrillation coils of the type discussed in the ""049 patent involves the occurrence of tissue in-growth around the coil structure of the electrode. This is particularly true when the coil electrode is formed around the lead body so as to create an enlarged profile at the location of the coil as compared to the rest of the lead body. Tissue attachment that occurs proximal to the electrode structure makes it difficult to extract distal end of the lead, including the enlarged coil.
One way to reduce the problem of tissue in-growth involves providing defibrillation coils that are isodiametric with respect to the lead body. Such isodiametric electrode coils may be molded into the electrode body or the coils may be machined to provide a flush surface. This is described in U.S. Pat. No. 4,161,952, issued to Kinney et al. Similarly, U.S. Pat. No. 5,957,970 to Shoberg discloses an isodiametric defibrillation lead manufactured by removing a portion of an extruded tubular lead body in the region of the coil so that the electrode is flush with the surface of the lead. The 6944 Model lead commercially available from the Medtronic Corporation provides an isodiametric lead of this design.
Although isodiametric coil electrodes reduce problems associated with extracting an enlarged lead portion from ingrown tissue, other problems still exist related to in-growth. For example, tissue commonly attaches around the coils of a defibrillation electrode, further increasing the difficulty associated with lead extraction.
Various methods have been attempted to overcome the problems associated with tissue in-growth around coil electrodes. One solution disclosed in the ""049 patent referenced above involves injecting silicone rubber into the spaces between the individual coils of an electrode. The resulting thin coating of silicone rubber surrounding the exterior of the coils of electrode minimizes tissue in-growth between the filars of the coils, while leaving a portion of the coils exposed to deliver electrical stimulation to a patient.
Another approach to preventing tissue in-growth is disclosed in U.S. Pat. No. 5,090,422, which describes the use of a biocompatible porous materials such as woven, porous polyurethane and porous polytetrafluoroethylene that may be used to cover an electrode surface. The material is insulative when dry, but becomes conductive when bodily fluids penetrate the pores of the material. The porous covering is of adequately small pore size and fibril length to preclude substantial tissue in-growth.
Yet another method of preventing tissue in-growth is disclosed in U.S. Pat. No. 5,609,622, which describes coating a lead with a porous Polytetrafluoroethylene (PTFE) layer such as may be formed of expanded PTFE (e-PTFE), and which has a pore size of less than 10 microns or smaller so that the pore size is very small, and tissue in-growth is prevented.
While the foregoing approaches are directed to preventing tissue in-growth, it may be noted that some controlled tissue attachment to a lead body or electrode may actually be beneficial in stabilizing the lead, and allowing the electrode to maintain a position at a desired implant site. For example, in leads carrying defibrillation electrodes, a change in the coil placement may increase the defibrillation thresholds. Similarly, in pace/sense applications, a shift in electrode position may alter the pacing threshold and affect capture. Promoting tissue growth for stabilizing prosthesis and other implanted structures has been disclosed in the prior art. For example, U.S. Pat. No. 5,035,713 discusses use of a re-entrant biocompatible material such as polyethylene, polyethylene teraphthalate, polypropylene, polysulfone, polylactic acid and polydioxanone to promote selective tissue in-growth to stabilize an implanted structure. Similarly, U.S. Pat. No. 5,833,664 discusses promoting tissue attachment to intrabody prosthetic devices, such as catheters, to effect improved stability of the prosthesis/tissue opening site interface.
Yet a further concern associated with the use of medical electrical leads, and in particular, the use of leads carrying coiled electrode structures, involves the possibility of a coil causing abrasion to an adjacent lead structures. For example, a coiled electrode adapted for use in the right atrium may lie in close proximity to a second lead carrying an electrode placed in the right ventricle. The coiled electrode may contact the second lead, creating abrasions in the external lead surface so that an internal conductor comes in contact with body fluids. This condition may ultimately result in lead failure. Also, in applications in which leads are positioned within relatively smaller vasculature structures, such as in coronary veins, lead-to-lead contact, or lead-to-tissue contact can be exacerbated, thus increasing the likelihood of abrasion.
Therefore, what is needed is an improved lead structure that minimizes the potential for the abrasion of adjacent structures. The lead ideally promotes selective, controlled tissue attachment to stabilize lead placement, while preventing tissue in-growth that would prevent lead extraction.
The present invention provides an improved system and method of selectively promoting tissue in-growth on, or adjacent to, an implantable medical device (IMD). In one embodiment of the invention, a first portion of porous PTFE is provided adjacent to at least a first surface of the IMD. The porous PTFE, which may be an expandable PTFE (e-PTFE), has a pore size that is small enough to prevent substantially all tissue in-growth. For example, the pore size may be 10 microns or less. This first portion of PTFE is adjacent at one or more selected locations to a second portion of porous PTFE. This second portion of PTFE has a pore size adapted to promote tissue in-growth. For example, the pore size of this second portion may be 20 to 50 microns. In this manner, tissue in-growth will only occur at the first surface of the IMD in the vicinity of the second portions of PTFE. At other areas of the first surface of the IMD, all tissue in-growth will be substantially prevented.
In another embodiment of the invention, the inventive system includes a removable member that is adapted to be adjacent to at least one surface of the IMD. The removable member is formed of a first portion, or layer, of porous PTFE to substantially prevent all tissue in-growth, and one or more second portions, or layers, of porous PTFE that are provided to promote tissue in-growth in selected locations. The removable member may take the form of a sleeve, as may be adapted to slide over a portion of a lead. This sleeve may be adapted to remain in the body of a patient during a lead extraction process so that a replacement lead may be advanced within the sleeve. Because the sleeve is adapted for preventing tissue in-growth in the lead itself, the lead extraction process is greatly simplified.
According to one embodiment of the invention, each of the PTFE layers are formed of porous PTFE tubing having a thickness of 50 microns or less, and preferably less than 25 microns. The tubing may be heat-shrinkable such that it conforms to a specific surface of an electrode or other implantable medical device upon the application of heat. In yet another embodiment of the invention, the layers of porous PTFE may be formed of a tape that may be applied to the surface of the implantable medical device. In still a further embodiment of the invention, the two layers are provided by a single composite structure that has a more porous material exposed on a first surface adapted to be located adjacent tissue, and a less porous, more dense material on a second surface adapted to be situated adjacent the electrode. This type of composite structure may be formed by first creating a coating of e-PTFE having a unified density. One surface of the composite structure may then be altered to be more porous by selectably removing fibrils from this surface. Alternatively, this structure may be formed using a co-extrusion process to create the layers with different pore sizes.
According to one manner of using the current invention, a lead having at least one coil electrode is coated with an inner layer of porous PTFE such as expanded-PTFE (e-PTFE) to prevent tissue in-growth. This inner layer is surrounded by a more porous PTFE layer having a pore size selected to promote tissue in-growth. The tissue in-growth promoted by the pore size of the outer layer is controlled such that the coil electrode is stabilized at a desired implant site by the tissue attached to this layer. Tissue is prevented from adhering to any part of the coil structure itself by the inner layer.
The current invention may be adapted for selectively promoting tissue in-growth in subcutaneous electrode arrays, and in any other type of implantable lead or electrode system. In addition to promoting tissue in-growth to stabilize the location of an implantable medical device, the current system also prevents lead abrasion cause by electrode coils rubbing against adjacent lead structures when multiple leads are in close communication with one another within a patient""s vascular system.